Recently, an indirect heated cathode is used for an electron tube such as a TWT (travellingwave tube) an a klystron. The cathode is usually heated by a heater coil positioned thereunder. In the electron tube, when the cathode is heated to a predetermined temperature, thermionics are emitted from an emission face of the cathode.
A first conventional heater coil of a side-cross-single-spiral type is made of a single wire including a spiral wire double wound to be cylindrical and lead wires extending to a vertical direction for being connected to a power supply. The heater coil is provided with a U shaped bent portion at a top thereof.
In the conventional electron tube, when a predetermined voltaghe is applied to the lead wires, the cathode pellet is heated, and thermionics are emitted from the emission face of the cathode pellet.
A second conventional heater coil of top-cross-single-spiral type is made of a single wire including a spiral wire double-wound to be cylindrical, a connection wire extending in a diametral direction of the spiral wire, and lead wires connected to a power supply.
A third conventional heater coil of top-cross-double-spiral type is shaped to be top-cross-single-spiral shape in the same manner as the second conventional heater coil by using a spiraled wire.
According to the first to third conventional heater coils, however there is a disadvantage in that electrons emitted from the cathode are disturbed by a magnetic field generated to be changed around the heater coil heated by an AC power supply. Therefore, a noise is included in an output signal amplified by the electron tube.